1. Field of the Invention
The present invention relates to temperature control circuits that use a processor to execute a heating program for heating a product in a heating chamber wherein the processor is subject to unintentional reset during execution of the heating program from electromagnetic interference.
2. Description of Related Art
A heating chamber, such as an oven, that is heated by a gas fired fuel source requires a means of initially igniting the gas. Ignition of the gas can be accomplished with an electric spark igniter. The igniter generates an electric spark across a gap in the presence of gas to ignite the gas. A temperature control circuit is used to establish a heating or cooking program for the heating chamber. The heating process is executed with a processor having appropriate input and output interfaces. The generated electric spark can radiate an electromagnetic field that will interfere with electric circuitry in its vicinity. Such interference is generally called electromagnetic interference (EMI). FIG. 1 diagrammatically illustrates a typical heating chamber 10 that uses a thermistor 6 as a temperature sensing element in the chamber and an electric spark igniter 5 to ignite gas supplied from an inlet 7 connected to a gas supply. Spark igniter controller 9 controls a gas control valve 11 that opens to release gas to outlet 13 when the igniter controller generates a high voltage across the spark gap of igniter 5. The resulting electric spark across the gap ignites the released gas. Temperature control circuit 15, powered from a suitable power supply 17, transmits a signal to the spark igniter controller that initiates the gas ignition process. Temperature control circuit 15 may also serve the function of a heating program controller that controls the temperature in the chamber for pre-determined time periods during the execution of the heating program. For example, a particular product may require that the chamber have a set temperature of 400xc2x0 F. for 20 minutes, and then a set temperature of 250xc2x0 F. for 40 minutes to achieve optimum heating of the product. As illustrated in FIG. 1, the temperature control circuit can also power and control one or more fan blower motors 19 for circulated convection heating in the heating chamber 10. The fan blowers may be operated in a pulse mode that turns the fan on and off for pre-determined (pulse) time periods. FIG. 2 illustrates a prior art temperature control circuit 25 that is used in a gas oven with an electric spark igniter. The typical temperature sensor, thermistor 6, exhibits a decreasing resistance as the temperature rises in heating chamber 10. A voltage divider is formed by supplying current from a source voltage reference VREF through resistor R7 in series with the thermistor. Capacitor C6, and resistors R2 and R3 form an R-C circuit that conditions the analog voltage input signal to the analog-to-digital (A/D) converter 20. The analog voltage input signal will change as the temperature in the chamber changes. Converter 20 outputs to processor 22 a digital signal on line 21 that represents the temperature in the chamber. Although a single line is shown, the digital signal may be transmitted on serial or parallel lines. Processor 22 uses the digital signal during execution of a heating program. Processor monitor 24 issues a reset signal to processor 22 on line 27 if a strobe input signal is not received from processor 22 on line 23 before a pre-determined watchdog time-out period has elapsed.
During execution of a heating program, the igniter 5 may re-ignite the gas a number of times following closure of control valve 11 after a set temperature has been reached in the chamber. During ignition, the electric spark generated across the gap of the igniter creates a broadband electromagnetic radiated pulse that is received by the thermistor 6 and propagated in the temperature control circuit. The associated electrical energy pulses in the conductors from the spark igniter controller to the igniter, and the circuitry of the spark igniter controller can also contribute to radiated EMI that is picked up by the temperature sensor. Unless adequate filtering is provided, components of the pulse will be injected into the input of the A/D converter and coupled into the power and ground voltages of all circuitry. The radiated EMI can also generate spurious signals on line 27 that can unintentionally reset the processor. Consequently, the temperature control circuit in FIG. 2 is susceptible to malfunction when the igniter generates a spark due to EMI.
An approach to solving to this problem is to use a voltage clamping circuit across the incoming lines from the thermistor to the temperature control circuit. However, a conventional clamping circuit, such as a zener diode or back-to-back diodes, still results in significant leakage at all voltages particularly when the thermistor has a large resistance range, such as 0 to 100 kilo-ohms (kohms). The leakage results in a non-linear error when reading the voltage, which is a function of temperature, across the thermistor. Because the error is non-linear and varies from sample to sample, it cannot be calibrated out.
Therefore, there exists the need for a temperature control circuit that will minimize the EMI effect on the circuit when it is used with a gas-fired heating chamber employing an electric spark igniter.
Total elimination of an unintentional processor reset in a temperature controller cannot be achieved. In addition to other sources of EMI, a temporary power failure during execution of a heating program will cause a processor to unintentionally reset. Time is lost in reinitiating a heating program after a processor reset. The problem can be solved by using an energy storage device such as a battery to retain power to components associated with storing and executing the cooking program. However, an energy storage system introduces a significant cost penalty and requires periodic replacement of the energy storage device.
It is another object of the invention to provide an efficient method of storing incremental cooking parameters during execution of a heating process so that if an unintentional reset occurs during the execution of a heating process the last set of valid cooking parameters can be recovered to continue operation of the process from the point before the reset.
In one aspect, the invention is a temperature control circuit for use with a heating chamber having a temperature sensor. The circuit includes a first common mode choke with the temperature sensor connected across the line terminals of the first common mode choke. The analog input of an analog-to-digital converter is connected across the line terminals of a second common mode choke. An R-C circuit may be provided between the analog input of the converter and the line terminals of the second common mode choke. The first terminals of a pair of output terminals for the first and second common mode chokes are connected together at a positive polarity line, and the second terminals of the pair of output terminals for the first and second common mode chokes are connected together at a negative polarity line. A reference voltage is suitably connected to the positive polarity line. One or more light emitting diodes (LEDs) are connected together in series, with their anodes oriented to the positive polarity line, between the positive and negative polarity lines to provide a suitable clamping voltage that is greater than the reference voltage but less than the maximum voltage for the analog input to the analog-to-digital converter. A diode or LED is connected anti-parallel across the series connected LEDs. Additional capacitive filtering can be provided between the positive and negative polarity lines, and between each line and ground. Additional capacitive filtering can also be provided between the analog input to the converter and ground.
In another aspect, the invention is a method of recovering from an unintentional processor reset during the execution of a heating process to heat a product in a heating chamber. Valid heating state parameters are incrementally stored at fixed time intervals in a nonvolatile memory device during execution of the heating process. After the heating process is interrupted by an unintentional reset, the incrementally stored valid heating state parameters that were stored at the last fixed time interval prior to the interruption are read from the nonvolatile memory device, and the heating process resumes at a point determined from the last read incrementally stored valid heating state parameters. Prior to resuming the heating process, the current temperature of the heating chamber can be compared with the temperature of the heating chamber from the last stored valid heating state parameters. If the difference between the current temperature of the heating chamber and the temperature read from the last stored valid heating state parameters exceeds a limit, the execution of the heating process can be terminated.
These and other aspects of the invention will be apparent from the following description and the appended claims.